Natural durability of timber species exposed to xylophagous fungi in southern Durango, Mexico

Introduction Wood is a natural resource used for construction and the manufacture of many products. This material is exposed to damage due to biotic and abiotic factors. An important biotic factor is wood-degrading fungi that generate large economic losses. The objectives of this study were to determine the effect of xylophagous fungi (Coniophora puteana and Trametes versicolor) on the natural durability of six timber species in southern Durango, Mexico, and to establish differences between fungal effects on each tree species. Materials and Methods Samples of Pinus durangensis, P. cooperi, P. strobiformis, Juniperus deppeana, Quercus sideroxyla, and Alnus acuminata were exposed to fungi for 4 months under laboratory conditions according to European Standard EN350-1. Samples of Fagus sylvatica were used as control. Durability was determined as the percentage of wood mass loss for each species. Welch ANOVA tests were performed to establish differences among tree species. Welch t-tests were used to prove loss mass differences between fungi for each tree species. Results The most resistant species to C. puteana were P. durangensis, J. deppeana, P. cooperi and P. strobiformis, showing mean mass losses lower than 8.08%. The most resistant species to T. versicolor were J. deppeana, P. strobiformis and P. durangensis (mean mass losses lower than 7.39%). Pinus strobiformis and Q. sideroxyla were more susceptible to C. puteana effect; in contrast, P. durangensis and P. cooperi showed more damage due to T. versicolor degradation. Conclusions Woods of P. durangensis, P. cooperi, P. strobiformis and Juniperus deppeana are well adapted to infection by these xylophagous fungi and are therefore highly recommended for commercial use in southern Durango, Mexico.


INTRODUCTION
Wood is a renewable natural forest product that has high economic importance as it serves as a material for construction and the manufacture of a large number of objects (Vázquez, 2014). Durability is the resistance of wood to degradation by abiotic and biotic factors and depends on the tree species and the care that is given to the wood (Colín-Urieta et al., 2015). Abiotic factors such as changes in temperature, high humidity, aerial erosion, rain, and ultraviolet rays cause photodegradation, swelling, cracks, and discoloring of the wood (Trevisan et al., 2008;Torres, Figueroa & Vives, 2011;Hibbett et al., 2016). Biotic factors also affect wood quality.
With respect to biotic factors, fungi are among those that most deteriorate commercial wood (Miklósiová, 2014;Sundararaj et al., 2015); these microorganisms are capable of degrading wood cell walls at different levels by using their components. The main groups of degrading fungi are called white, brown, and soft fungi due to the effect produced on wood. They generate rot in the wood by using chemical substances at different rates, thus producing different patterns of decomposition (Kubicek, 2012). Airborne spores are the main source of rot fungi infection in above ground conditions (Zabel & Morrell, 2012). After prolonged moisture exposure, spores can trigger an infection in unprotected wood (Deacon, 1997). The fungi able to grow in sapwood and heartwood are different; this indicates the influence that different levels of moisture absorption, intermittent warming, condensation processes, or naturally occurring antifungal substances may have. Species that can tolerate the concentration of tannins or other polyphenols can become established in the heartwood (Råberg et al., 2005).
Broadly speaking, fungal development in wood is favored by conducive humidity (60%), pH (alkaline), and temperature (20 C) levels, as well as the amount of starch and sugars in the cell walls; then fungi establish in plant tissues and feed on organic substances. Wood degradation due to fungi is observed as a dark stain on the sapwood that causes a decrease in quality, thereby decreasing the market price (Zabel & Morrell, 2012;Peraza-Sánchez, 2002). Fungal enzymes weaken the wood by breaking down cellulose and lignin in the cell walls; wood damage depends on the fungal species and the type of compound that each one degrades (Cruz de León, 2010).
Due to their importance as wood destroyers and their wide distribution, some fungal species have been used as a test fungus for wood preservatives in European trials for over 70 years (Kauserud et al., 2007). In this regard, Coniophora puteana is a very common threat to wooden buildings and other structures where softwood is used (Hiltunen et al., 2020); the damage it causes has been evidenced more frequently indoors in the so-called subtropical climates of southern European countries (Bogusław Adam, Krajewski & Betlej, 2021). This species is distributed in different forms and in various types of dead wood throughout the world. According to Ramsbottom's research (Ramsbottom, 1937), a group of fungi, including C. puteana, caused enormous rot problems on sailing ships during the 18th century which may have led to the dispersal of these fungi around the world (Kauserud et al., 2007). These fungi produce brown rot, which mainly breaks down cellulose and hemicellulose in wood and leaves modified lignin as a residue, and it can cause a loss of wood mass of up to 70% reducing mechanical resistance and causing dark staining in wood (Hiltunen et al., 2020;Brischke et al., 2013;Hyde & Wood, 1997). On the other hand, according to Karim et al. (2017), Trametes versicolor causes more than 90% of white-rotting damage to trees and wood in northern Iran and North America (Bari et al., 2016).
This type of fungi degrades lignin, which affects the strength and compression of the wood; this degradation is called white rot because of its fibrous or even mealy appearance (Luley, 2005).
In Durango, Mexico, there are more than 20 species of timber trees, among which are the genera Pinus spp., Quercus spp., Juniperus spp. and Alnus spp. (García & González, 2003). They are widely distributed across the Sierra Madre Occidental and have high economic importance (SEMARNAT, 2010). The wood of these species is exported in the form of sawnwood, venner sheets, plywood, particle boards, OSB boards and MDF boards mainly to the United States and Canada, European Union countries such as Germany, Spain and the United Kingdom, and Asian countries such as Japan, South Korea and China (FAOSTAT, 2020;Torres-Rojo, 2021;Yuan & Tsigas, 2018;Velarde-Moreno, Alarcón-Osuna & Blanco-Jimenez, 2019). However, to date there is little information on the natural durability of Mexican timber species to xylophagous fungal infections. By increasing technological knowledge and their natural durability, the use of these species could be expanded. Due to the above, the objectives of this study were to determine the effect of xylophagous fungi (Coniophora puteana and Trametes versicolor) on the natural durability of six timber species (Pinus cooperi, P. strobiformis, P. durangensis, Alnus acuminata, Juniperus deppeana and Quercus sideroxyla) in southern Durango, Mexico, and to evaluate differences between fungal effects for each tree species. Samples of Fagus sylvatica were used as the control. This information is expected to increase the knowledge needed to support forest management measures that help prevent economic losses related to fungal degradation in Durango, Mexico.

Sawing and dimensioning of specimens
For each tree species, logs (one meter in length and 30 cm in diameter) were first collected and then immediately sawn to make boards, whose dimensions varied depending on the species. Boards were air dried for a month, under temperatures of 1 C to 20 C and relative humidity of 64%. A total of 420 samples (3 cm long, 1 cm wide and 0.5 cm thick) were obtained from all the boards, corresponding to 60 samples (30 for each of the two fungal species) for each of the seven timber species. The rings were considered to have an angle between 30 and 70 according to European Standard 113-1 (CEN EN 113, 1996).

Wooden sample drying and conditioning
Samples were dried in a stove at 103 C until constant weight was reached; then dry mass was determined on an analytical balance to the nearest 0.01 g. Finally, the samples were conditioned to 20 C and 65% relative humidity to achieve a moisture content of 12% as is normally found in wood in service.

Evaluated treatments
Petri dishes containing 0.50 mL of malt agar were prepared. A one cm inoculum of Coniophora puteana was placed in each of three Petri dishes. Five wooden samples of Pinus cooperi and one of Fagus sylvatica were placed around the inoculum at an equal distance of separation. Another three Petri dishes were prepared placing the six wooden samples but without inoculum, serving as controls (leaching). The same was done for each of the seven wood species (a total of 42 Petri dishes). Subsequently, this same experimental design was applied using Trametes versicolor as inoculum. For 4 months the Petri dishes were kept under controlled conditions in an incubator (23 C and 65% relative humidity). Each month the Petri dishes were monitored to avoid contamination from other fungal species.

Quantification of mass loss
At the end of the growth period of the fungus, the anhydrous weight of the wooden samples of all tree species was determined. Mass loss was estimated using the following formula: where: Pm = Mass loss (%); Pi = Anhydrous mass of the specimen at the beginning of the test (g); Pf = Anhydrous mass of the specimen at the end of the test (g).

Classification of natural durability
The natural durability of each species was expressed according to the "X value", which was obtained by dividing the average mass loss of each species by the mass loss of F.

Statistical analysis
Descriptive statistics of mass loss for each tree species were obtained. Shapiro-Wilk normality and Levene homogeneity of variance tests were performed. Data were normalized by transforming to log10+1 (Zar, 1999). Since the data showed heterogeneity of variance, Welch ANOVA tests were used to observe differences of mass loss among tree species; Games-Howell tests were used to separate groups. Also, Welch t-tests were used to prove mass loss differences between fungi for each tree species. All tests were considered significant at P < 0.05 and were conducted in SPSS Ver.18.

RESULTS
Durability of wood exposed to Coniophora puteana Significant differences in mass loss were observed among the tree species exposed to C. puteana (Welch = 240.78, df = 6, 43.10; P < 0.0001; Table 1, Fig. 2). Although this fungal species is considered to deteriorate softwood speces to a greater extent, in this study the highest mass losses were in Alnus acuminata, Q. sideroxyla and the control F. sylvatica, which are in the same statistical group. On the contrary, Pinus durangensis showed the lowest mass loss (x = 2.65%), followed by J. deppeana (x = 4.88%). Pinus cooperi and P. strobiformis, for their part, formed their own statistical group, both losing 8% of their mass.
Durability of wood exposed to Trametes versicolor Significant differences in mass loss were observed among the tree species exposed to Trametes versicolor (Welch = 260.43, df = 6, 43.26; P < 0.0001;

Differences in mass loss among timber species for each fungus
The deterioration caused by fungi C. puteana and T. versicolor was high; some sections of the timber species A. acuminata, P. cooperi, and the control F. sylvatica were totally degraded (Fig. 4). According to the Welch t-test, Pinus strobiformis and Q. sideroxyla were significantly more susceptible to C. puteana damage; in contrast, P. durangensis, P. cooperi and the control (F. sylvatica) showed more damage due to T. versicolor (Table 3). Juniperus deppeana and A. acuminata showed a non-significant difference between fungal degradation effects (Table 3).

DISCUSSION
Both species of fungi caused different levels of deterioration in the wood samples analyzed, as can be seen in Fig. 4 with A. acuminata, P. cooperi, and the control species F. sylvatica.

Differences in mass loss among timber species for each fungus
The timber species analyzed in this study showed statistically significant levels of durability, determined by the mass loss caused by the attack of the wood-degrading fungi Coniophora puteana and Trametes versicolor (Figs. 2 and 3). The results of this work are validated according to the mass loss of Fagus sylvatica used as the control, which was greater than 20% for both wood-degrading fungi; this value was established by European standard 350-1 (CEN EN 350-1, 1994). No P. sylvestris wood samples were used to validate the deterioration in softwood species; however, P. durangensis, P. strobiformis and P. cooperi species showed loss values from 2.65% to 18.92%. In general, these three species and J. deppeana showed from medium to higher resistance to deterioration due to the action of fungi compared to Q. sideroxyla and A. acuminata, which were the most susceptible species to these xylophagous fungi. Similar results to those obtained in this   Ayata, Akcay & Esteves (2017) in an experiment where thermal treatments were used to increase the durability of Quercus petreae; they found mass losses of 21.54 and 11.20% by exposing this timber species to Pleurotus ostreatus and Coniophora puteana fungi, respectively. On the other hand, the mass loss caused by C. puteana in A. acuminata in this trial was higher than the 14.66% reported by Erazo et al. (2019); however, the values were lower than the 61.84% reported by the same authors when the wood was exposed to T. versicolor.

Classification of natural durability
When using the durability classification established by European standard EN 350-1 (CEN EN 350-1, 1994), it was found that Q. syderoxyla was not durable (Class 5) when in contact with C. puteana, while A. acuminata was slightly durable (Class 4), J. deppeana, P. cooperi and P. strobiformis were moderately durable (Class 3), and P. durangensis was very durable (Class 1). Using the same classification system, but for T. versicolor as a deterioration agent, A. accuminata, P. cooperi and Q. sideroxyla were classified as moderately durable (Class 3), while P. durangensis and P. strobiformis were durable (Class 2) and J. deppeana was very durable (Class 1). The low mass loss (Classes 3 and 1 when exposed to C. puteana and T. versicolor, respectively) of J. deppeana wood could be the result of a high content of extracts (Kirker, Bishell & Lebow, 2016). These non-structural wood components are often concentrated in different proportions in a radial direction and along the trunk of the tree, thereby avoiding biological attacks as established by Erazo et al. (2019). In Oregon, USA, Juniperus wood samples have shown many years of durability (>50), which is attributed to their extracts that provide a high inhibitory effect on the growth of Gloeophyllum trabeum and T. versicolor (Mun & Lynn, 2011). In contrast, several tree species produce non-toxic extracts that do not reduce damage by xylophagous fungi (de la Cruz Carrera et al., 2018).
Regarding the timber species Alnus acuminata and Q. sideroxyla (Classes 4 and 5 when exposed to C. puteana and Class 5 when exposed to T. versicolor, respectively), their resistance ranged from slightly durable to not durable, with the composition of their extracts possibly being the cause. Similar studies conducted in Mexico found that T. versicolor caused a greater loss of mass (30.7%) in samples of Arbutus spp. and Q. sideroxyla; on the other hand, J. deppeana, P. strobiformis, P. durangensis, P. teocote and P. cooperi showed low mass losses (0.97%), placing these woods in Class 1, which corresponds to very durable (de la Cruz Carrera et al., 2018). The natural durability of the Mexican timber species tested in the present experiment was within or above the reported values for the yellow pine, Quercus petreae, Pinus caribaea and Eucalyptus saligna species (Fojutowski et al., 2014;Lana & Bernardes, 2019), which may give them an opportunity to be marketed in other countries where natural durability is an important selection factor.

CONCLUSIONS
It is concluded that J. deppeana, P. durangensis, P. strobiformis and P. cooperi can be used in southern Durango, Mexico, in conditions of exposure to xylophagous fungi, in direct contact with the soil. However, in Mexico there is a need for more information and a better understanding about the process of microbial colonization and succession of wood. Also,